Aluminium High-Voltage Power Transmission

Here’s a compendium of factoids on power transmission, with a little directly on skin effect (which may not be the only salient aspect). I believe that skin effect per se is due to AC. Since not all long lines are AC, and since those that are AC are only 60Hz, skin effect doesn’t much enter into it. The real explanation for the practical use of what we think of as crummy aluminium* wire emerges from the following factoids (below the fold).

I had been talking with some radio & engineering friends, and we were surprised that aluminium cables are used for long-distance power transmission lines—even those which use Direct Current (DC). In fact, a few of us were surprised that long-distance power transmission can even be done with DC. So I did some online research, and learned a bunch of things. Of course, I’m not an EE, so don’t go running aluminium long-distance DC power transmission lines without professional help.

Power transmission line factoids:

1. Aluminium is cheap.

2. Decent-conducting alloys are possible.

3. Increasing the voltage can lower the current so much that resistance isn’t much of a problem.

Power generation is usually at 60Hz triple phase between 2,300V and 30,000V. Triple phase is used because it makes best use of 3-wire lines; dual phase also requires (unbalanced) 3-wire lines. In rural areas, sometimes single-wire lines are used with Earth return (pity the nematodes and termites).

Efficient long-distance transmission is ballparked at a 4,000 mile limit. The world’s longest lines rarely exceed about 1,000 miles, mainly in the Congo and in Siberia.

Frequency is not stepped up for the national grid, but voltage, of course, is. Sometimes DC is used for significant amounts of power and distance (see below), with certain advantages over AC.

Joule’s Law states that the resistance goes down by the inverse square of the current, so by doubling the voltage you halve the current and reduce the resistance losses by 4.

Wire for long distance transmission is usually stranded aluminum, often with a steel core for tensile strength; copper doesn’t improve conductance due to the already low current of the high voltage. Thermal effects become significant only at very high loads, and some new lines embed fiber optics in the transmission line so that changes in optical reflectance can be used to estimate locations of unacceptable heating. The wires stretch and contract enough to create changes in ground inductance and vulnerability to wind motion. The highest capacity currently is at Iguazu Falls in Paraguay/Brazil (which is also the world’s largest hydroelectric plant); the individual lines from Itaipu plant deliver about 6MW at 600kV.

High voltages for power transmission fall in the range of 110kV up to as much as 2MV. Lower voltages (66kV and 33kV) are used for shorter hauls or light loads. Above 230kV special step-up and step-down equipment is required. Upper ranges around 750kV-1,200kV are most common among the extra high voltage lines, but above one million volts corona discharge begins to become a factor and around two million volts corona produces significant losses.

High voltage is also used for underground power transmission, although obviously there are a lot of side issues relating to inductance, capacitance, insulation, dielectric properties of materials, etc., and of course the installation cost. Exotic polypropylenes are currently used.

Transmission loss runs around 7.2% (US DOE) with about 60% from lines and about 40% from distribution transformers. So the figure of 50% loss you may have read about might have come from a misinterpretation of that information (about half of the loss is from the wires, but the total loss is only about 7% of the input power).

After distribution from the high voltage grid, regional distribution is usually in the 33kV to 66kV range. Local distribution to the pole-top step-down transformers is usually in the 3.3kV to 25kV range.

High-voltage long-distance DC transmission is also fairly widely used, especially to share power between two grids whose generators are not in phase, and occasionally when capacitance is a problem. DC transmission lines are not subject to the phase-related losses of AC lines.

One interesting hypothesis concerning the slight evidence of high EMF being bad for your health is that the electric component of the EMF may attract and concentrate airborne pollutants. I find this intriguing since it doesn’t involve mechanisms that are still almost totally unknown.

Living under high-voltage power transmission lines is definitely not recommended.

End of transmission.

__________________
*Yup, aluminium is now officially spelled with that “ium” ending, for consistency with the other metals in its column of the Periodic Table. So it’s no longer (technically) a Britishism.

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One Response to Aluminium High-Voltage Power Transmission

  1. Mister Natural says:

    When the density of Cu (559 lb/ft^3) is compared
    to that of Al (169 lb/ft^3) and taking into
    consideration the conductivity ratio of Al to Cu of
    56%, the result shows that on a pound per pound
    basis, Al has an amperage capability that is
    approximately 1.85 times that of Cu. In other
    words, one pound of Al has the same electrical
    capability as 1.85 pounds of Cu. Cu has a greater
    conductivity on an equal volume, cross sectional
    area, basis.

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